1. Field of the Invention
The present invention relates generally to an apparatus for sensing the roll rate of an item such as a spinning projectile. More specifically, the present invention relates to a magnetic roll rate sensor for sensing the roll rate or roll position of a missile which allows for a determination to be made of the missile""s position in earth coordinates.
2. Description of the Prior Art
Currently, there is a need to measure the roll rate or roll position of a spinning projectile such as a missile. Measuring the roll rate or roll position of a rolling airframe, such as a missile can be very difficult, especially with respect to a smaller diameter missile. Some of these smaller diameter missiles have roll rates in excess of 20 revolutions per second. The diameter of such a projectile may be in the order of approximately 2.75 inches leaving very limited space for the roll rate sensor. Thus, bulky roll rate sensors, such as laser ring gyros and quartz rate sensors are not easily adapted for use in small diameter missiles.
The high roll rates of these small diameter missiles also cause position errors to occur during the missile""s flight. For example, a quartz rate sensor measuring roll would require a range of xc2x17200 degrees to cover a missile having a roll rate of about 20 revolutions per second. Scale factor accuracy is typically about xc2x1 one percent of full scale for a quartz rate sensor. Thus, the position error for a ten second missile flight is xc2x1720 degrees (xc2x172 degrees per second times ten seconds). Added to this error are other error sources such as bias, linearity and g sensitivity.
Further, the cost of these roll rate sensors is significant, driving up missile production cost. Accordingly, there is a need for a smaller, less expensive, light weight, and very accurate roll rate sensor which measures the roll rate. or roll position of a missile in flight.
The roll rate sensor disclosed herein overcomes some of the disadvantages of the past including those mentioned above. It comprises a relatively simple in design yet highly effective and efficient device for measuring the roll rate or roll position of a small diameter projectile such as a missile.
A pair of magnetic sensor elements mounted within the missile provide analog electrical signals which indicate a change in the earth""s magnetic field due to the presence of a ferromagnetic element within the field. The analog signals, representative of roll rate sensor data, are converted to a digital format, formatted into a PCM data stream and transmitted to a ground station via an RF (radio frequency) signal link in the form of a radio frequency signal. The ground station receives the PCM data stream, extracts the roll rate sensor data and processes the roll rate sensor data using a personal computer. The personal computer includes computer software which performs quadrature detection to determine roll rate for the missile.